Ligand Bond Distances

Geometries of Coordination Polyhedra around Some Transition Metal Cation  

Metal—Ligand Distances Measured in Crystal Structures of Small Molecules 

Bond Coordination number (c) Average bond length (A) Reference  

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As briefly stated in the introduction, we may consider one-dimensional cross sections through the zero-order potential energy surfaces for the two spin states, cf. Fig. 9, in order to illustrate the spin interconversion process and the accompanying modification of molecular structure. The potential energy of the complex in the particular spin state is thus plotted as a function of the vibrational coordinate that is most active in the process, i.e., the metal-ligand bond distance, R. These potential curves may be taken to represent a suitable cross section of the metal 3N-6 dimensional potential energy hypersurface of the molecule. Each potential curve has a minimum corresponding to the stable... 

It may be noted that, as expected, the equatorial distances are shorter than the axial. There is, of course, a close relationship between the square-pyramidal and the F-trigonalbipyramidal coordination sphere which are often difficult to discriminate when the tin-ligand bond distances in equatorial and axial positions do not differ significantly. 

X-ray structural studies of the diamagnetic anion (406) confirm that the Ir(-I) center is in a distorted coordination geometry intermediate between square planar and tetrahedral, with the P donor atoms in a cis position. The metal-ligand bond distances do not show significant changes among (404), (405), and (406). The Ir1/0 and Ir0/(-1) redox couples are measured at easily accessible potentials and are solvent dependent. 

In addition, the determination of metal-ligand bond distances in solution and their oxidation state dependence is critical to the application of electron transfer theories since such changes can contribute significantly to the energy of activation through the so-called inner-sphere reorganizational energy term. 

As the flexibility of the macrocycle increases, then mismatch hole-size effects are expected to be moderated. In any case, as discussed in Chapter 1, a metal ion which is too large for the cavity may be associated with folding of a flexible macrocycle thereby allowing normal metal-ligand bond distances to be achieved. However, this is not always the case, and a number of examples of unfolded macrocyclic complexes containing compressed metal-donor distances are known (Henrick, Tasker Lin-doy, 1985). 

The crystal structure of poplar apoplastocyanin has been reported at 1.8 A resolution [42]. The structure closely resembles that of the holoprotein, and positions of the Cu-binding residues are different by only 0.1-0.3 A. Tetrahedral coordination, with proportionately larger metal-ligand bond distances, is also observed on replacing the Cu by Hg(II) [43]. This suggests that the irregular geometry of the active site is imposed on the metal atom by the polypeptide... 

Amino Acid Ligand Bond Distance Amino Acid Ligand Bond Distance (A)... 

Table 5-10 Metal-Ligand Bond Distances in Transition-Metal Coordination Compounds... 

Table 5-14 Metal-Ligand Bond Distances in Second and Third-Row Transition-Metal Organometallics... 

See, R. F., Kruse, R. A., and Strub, W. M. (1998). Metal-ligand bond distances in first-row transition metal coordination compounds Coordination number oxidation state and specific ligand effects. Inorg. Chem. 37, 5369-75. 

The sulfur atoms are also clustered in all-cis positions in the txtxp2p2 bicapped trigonal prismatic structure of tetrakis(A-methyl-p-thiotolylhydroxamato)hafnium(IV) [HffMeQJlr C(S)N Oj Me 4]. Averaged metal-ligand bond distances are Iff—0 = 2.150 and Hf—S =... 

Table 2.5. The influence of 1,3-nonbonded interactions on the ideal metal-ligand bond distance of chro-mium(III), cobalt(III), and nickel(II) hexaamines.

Another factor comes into play here. In the first coordination sphere, deviations in metal-ligand bond distances are normally small enough to be neglected. The nearest 14 K+ ions lie at seven separate distances, from 4.3 to 6.0 A. The variation in the magnitude of the perturbation with distance must be considered, and it is not straightforward. Classical crystal field theory shows an inverse fifth power dependence of Dq on distance from the metal, but this result is specific to octahedral symmetry, in which lower order dependencies drop out. For individual perturbers we have proposed a dependence of the form aR-3 + bR-5 for both e and e [7]. For counterions we also suggest that the n contribution and the R-5 part of the a contribution can be neglected. 

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